Abstract

The Josephson effect is a privileged access to the macroscopic quantum nature of superconductors. We review some ideas and experimental techniques on macroscopic quantum decay phenomena occurring in Josephson structures. The attention is mainly addressed to intermediate levels of dissipation which characterize a large majority of low critical currentJosephson devices and are therefore an avoidable consequence of nanotechnology applied more and more to Josephson devices. Phase diffusion phenomena take over thermal activation in some temperature ranges also affecting the transition to macroscopic quantum tunneling, enriching the phase diagram mostly defined by the Josephson energy, the temperature and the level of dissipation.

This contribution initially intended to celebrate 50 years from the discovery of the Josephson effect, also stands now as the last paper of our friend and maestro Antonio Barone. Antonio passed away before he could see the final line of the work, that however responds to the common initial planning. This manuscript is now also in his honor, touching one of his favorite topics, macroscopic quantum phenomena in Josephson junctions. This work is supported by MIUR PRIN 2009 under the project SuFET based on nanowires and HTS. We also acknowledge partial support by STREP MIDAS, Macroscopic Interference Devices for atomic and Solid State Physics: Quantum Control of Supercurrents and by a Marie Curie International Reintegration Grant No. 248933 hybMQC within the 7th European Community Framework Programme.

Article outline:I. INTRODUCTIONII. A BALL ROLLING DOWN A WASHBOARD POTENTIAL: FROM THERMAL ACTIVATION TO MACROSCOPIC QUANTUM TUNNELINGA. Formalism and the first measurements on switching current distributionsB. Josephson junctions as a part of a qubitIII. PHASE DYNAMICS IN “NOVEL” TYPES OF JUNCTIONS AND THE MODERATELY DAMPED REGIMEA. Macroscopic quantum phenomena in HTS grain boundary Josephson junctionsB. Macroscopic quantum phenomena in HTS intrinsic junctionsC. The moderately damped regime in low critical currentsJosephson junctionsD. In the “far” low critical current regimeE. Switching current distribution measurements in “nanostructures”IV. CONCLUSIONS